This invention relates to ultrasound imaging systems and, more particularly, to a beamforming method and apparatus for such systems.
In single transducer ultrasound imaging systems, the ultrasound beam can only be focused conveniently at a fixed point in the field of view. However, multiple transducer ultrasound imaging systems, i.e. systems that use a linear array of transducer elements, permit the beam to be electronically focused at different depths in the field of view.
Various techniques are employed to phase the component signals received by the respective transducer elements of an array to form a resultant focused beam. For example, Maslak U.S. Pat. No. 4,140,022 discloses mixers individual to the transducer elements and a master delay line. The output of a local oscillator is applied with selective phase shifts to the mixers and the mixer outputs are selectively applied to the taps of the delay line so as to combine coherently the echoes received by the individual transducer elements. The delay line taps are close enough to cause "reasonable" phase coherence and the phase shifts introduced by the mixers and a selectively phase shifted local oscillator complete the focusing function. Each time it is desired to change focal zones, a new set of local oscillator phase shifts is selected and in some cases new delay taps are selected. As echos of one transmitted ultrasound burst are received from increasing depths of the field of view, the suitable focal zones for such depths are selected on a real time basis. Thus, the received echos are processed in a "dynamic receive focus" mode. Although Maslak teaches that the delay line taps can also be changed with each focal zone change, it is contemplated that the delay line taps will not have to be changed so often so as to avoid the use of expensive tap selector switches required to reduce noise.
Yamaguchi U.S. Pat. No. 4,392,379 discloses a pair of phased array circuits for dynamically focusing the echos received by a transducer array in an ultrasound imaging system. Each phased array circuit has a delay line with taps to which the transducer elements are connected by a group of change overswitches. The change-over switches are wired to the delay line taps so as to focus the echos received from different zones in the field of view of the system depending on the switch setting. The two phased array circuits are alternately operated and reset, i.e., while one circuit is operating, the switches of the other circuit are being set. The delays required to focus the received echos depend on the characteristics of the transducer array head. Therefore, the change-over switch wiring is specifically designed for the particular transducer array head with which the system is to be used.